object reference not set to instance of object. help me with this. thanX

int j = dtview.CurrentRow.Index;
                                string query = "DELETE FROM Accounts WHERE Account_No=(Select Account_No from Customer where Customer_CNIC=@Customer_CNIC)";

                                SqlCommand cmd = new SqlCommand(query, cn);
                                cmd.Parameters.AddWithValue("@Customer_CNIC", dtview.Rows[j].Cells[4].Value);
                                cn.Open();
                                cmd.ExecuteNonQuery();
                                cn.Close();
                                MessageBox.Show("Record Deleted Successfully", "Success Message", MessageBoxButtons.OK, MessageBoxIcon.Information);